Electron spins in Earth’s mantle

When a transition-metal compound is subject to high pressure, its electronic spin state can change, which in turn can change the compound’s material properties. That spin-state crossover is of geophysical relevance because of the iron-bearing minerals in Earth’s lower mantle. But the most abundant mantle mineral—Fe-bearing magnesium silicate perovskite (Pv)—is a challenge to study, since it contains three nonequivalent types of Fe atom: Not only can Fe replace either Mg or Si in the crystal lattice, but Fe replacing Mg can be either ferrous (Fe²⁺) or ferric (Fe³⁺). Experiments on spin states under pressure probe the electron configuration indirectly, via its effect on nuclear energy levels, so computational studies are necessary to connect experimental measurements with the correct interpretations. Last year, an experimental study of ferric Fe in Pv yielded results that were at odds with the computational studies to date. Now, Renata Wentzcovitch and colleagues at the University of Minnesota have verified the experimental results computationally and predicted their geophysical consequences. The researchers found that ferric Fe that replaces Si undergoes a spin-state crossover at a pressure somewhere between 40 and 70 GPa, equivalent to a depth between 1100 and 2000 km and consistent with the 50–60 GPa crossover pressure measured experimentally. Since that transition causes the unit cell to shrink in volume by about 1%, it has a significant effect on the mineral’s bulk modulus and thus on the speeds of seismic waves and on mantle convection. (H. Hsu et al., Phys. Rev. Lett. 106, 118501, 2011 .)—Johanna Miller